1. Field of the Invention
The present invention relates to a screen for treating a fibrous stock suspension suitable for producing a paper, cardboard or tissue or other fibrous material web, the screen having a plurality of screen perforations, the smallest cross sectional areas of which are approximately of the same size.
2. Description of the Related Art
Screens of this type are used preferably for wet-screening of fibrous stock suspensions in order to remove contaminants contained therein. As a rule, they are rigid and thereby differentiate themselves from flexible endless screens which are used in screen presses and paper machines. The characteristic of such a screen essentially results from the size, shape and number of screen perforations contained therein. As a rule, these are kept smaller than the substances which are to be screened out. Such screens are advantageously utilized, for example, in pulpers, secondary pulpers and sorters for the preparation of paper fiber suspensions, whereby they are intended to hold back contaminants. In applications which arise in particular in the paper and pulp industry, screens of this type should have screening properties which are attainable through round perforations between 1 and 30 millimeters (mm), depending on the coarseness of the stocks. Of course, one also strives to enable an as great as possible throughput through such screens, meaning that a volume as great as possible of non-rejected substances passes through the perforations. This can be supported in that as many perforations as possible are provided. In general terms, as large as possible an open screen area relative to the overall surface of the screen element is strived for.
An additional requirement is a relatively high rigidity against hydraulic pressure. Such screen elements are utilized in a production operation where occasional break-downs are encountered, which leads to varying and considerable pressure load on the screen elements. Since clogging cannot always be eliminated, high pressures, and with appropriately large surfaces, also high forces can definitely act upon the surface of such screen elements. These increased forces must be absorbed by the screen without any damage occurring.
What is needed in the art is to improve the screening effect and/or the throughput while ensuring the necessary stability.